Generation of a drug resistance profile by quantitation of mdr-1/P-glycoprotein in the cell lines of the National Cancer Institute Anticancer Drug Screen

Cadmium transport by mammalian ATP-binding cassette transporters

Cellular responses to toxic metals depend on metal accessibility to intracellular targets, reaching interaction sites, and the intracellular metal concentration, which is mainly determined by uptake pathways, binding/sequestration and efflux pathways. ATP-binding cassette (ABC) transporters are ubiquitous in the human body-usually in epithelia-and are responsible for the transfer of indispensable physiological substrates (e.g. lipids and heme), protection against potentially toxic substances, maintenance of fluid composition, and excretion of metabolic waste products. Derailed regulation and gene variants of ABC transporters culminate in a wide array of pathophysiological disease states, such as oncogenic multidrug resistance or cystic fibrosis. Cadmium (Cd) has no known physiological role in mammalians and poses a health risk due to its release into the environment as a result of industrial activities, and eventually passes into the food chain. Epithelial cells, especially within the liver, lungs, gastrointestinal tract and kidneys, are particularly susceptible to the multifaceted effects of Cd because of the plethora of uptake pathways available. Pertinent to their broad substrate spectra, ABC transporters represent a major cellular efflux pathway for Cd and Cd complexes. In this review, we summarize current knowledge concerning transport of Cd and its complexes (mainly Cd bound to glutathione) by the ABC transporters ABCB1 (P-glycoprotein, MDR1), ABCB6, ABCC1 (multidrug resistance related protein 1, MRP1), ABCC7 (cystic fibrosis transmembrane regulator, CFTR), and ABCG2 (breast cancer related protein, BCRP). Potential detoxification strategies underlying ABC transporter-mediated efflux of Cd and Cd complexes are discussed.

Overcoming multi drug resistance mediated by ABC transporters by a novel acetogenin- annonacin from Annona muricata L

ETHNOPHARMACOLOGICAL RELEVANCE

Multi-Drug Resistance (MDR), mediated by P-glycoprotein (P-gp) is one of the barriers to successful chemotherapy in colon cancer patients. Annona muricata L. (A.muricata), commonly known as soursop/Graviola, is a medicinal plant that has been traditionally used in treating diverse diseases including cancer. Phytochemicals of A.muricata (Annonaceous Acetogenins-AGEs) have been well-reported for their anti-cancer effects on various cancers.

AIM OF THE STUDY

The study aimed to examine the effect of AGEs in reversing MDR in colorectal cancer cells.

METHODS

Based on molecular docking and molecular dynamic simulation, the stability of annonacin upon P-gp was investigated. Further in vitro studies were carried in oxaliplatin-resistant human colon cancer cells (SW480R) to study the biological effect of annonacin, in reversing drug resistance in these cells.

RESULTS

Molecular docking and simulation studies have indicated that annonacin stably interacted at the drug binding site of P-gp. In vitro analysis showed that annonacin was able to significantly reduce the expression of P-gp by 2.56 folds. It also induced apoptosis in the drug-resistant colon cancer cells. Moreover, the intracellular accumulation of P-gp substrate (calcein-AM) was observed to increase in resistant cells upon treatment with annonacin.

CONCLUSION

Our findings suggest that annonacin could inhibit the efflux of chemotherapeutic drugs mediated by P-gp and thereby help in reversing MDR in colon cancer cells. Further in vivo studies are required to decipher the underlying mechanism of annonacin in treating MDR cancers.

Long Noncoding RNAs in Taxane Resistance of Breast Cancer

Breast cancer is a common cancer in women and a leading cause of mortality. With the early diagnosis and development of therapeutic drugs, the prognosis of breast cancer has markedly improved. Chemotherapy is one of the predominant strategies for the treatment of breast cancer. Taxanes, including paclitaxel and docetaxel, are widely used in the treatment of breast cancer and remarkably decrease the risk of death and recurrence. However, taxane resistance caused by multiple factors significantly impacts the effect of the drug and leads to poor prognosis. Long noncoding RNAs (lncRNAs) have been shown to play a significant role in critical cellular processes, and a number of studies have illustrated that lncRNAs play vital roles in taxane resistance. In this review, we systematically summarize the mechanisms of taxane resistance in breast cancer and the functions of lncRNAs in taxane resistance in breast cancer. The findings provide insight into the role of lncRNAs in taxane resistance and suggest that lncRNAs may be used to develop therapeutic targets to prevent or reverse taxane resistance in patients with breast cancer.

Extracellular Vesicles and Cancer Multidrug Resistance: Undesirable Intercellular Messengers?

Cancer multidrug resistance (MDR) is one of the main mechanisms contributing to therapy failure and mortality. Overexpression of drug transporters of the ABC family (ATP-binding cassette) is a major cause of MDR. Extracellular vesicles (EVs) are nanoparticles released by most cells of the organism involved in cell-cell communication. Their cargo mainly comprises, proteins, nucleic acids, and lipids, which are transferred from a donor cell to a target cell and lead to phenotypical changes. In this article, we review the scientific evidence addressing the regulation of ABC transporters by EV-mediated cell-cell communication. MDR transfer from drug-resistant to drug-sensitive cells has been identified in several tumor entities. This was attributed, in some cases, to the direct shuttle of transporter molecules or its coding mRNA between cells. Also, EV-mediated transport of regulatory proteins (e.g., transcription factors) and noncoding RNAs have been indicated to induce MDR. Conversely, the transfer of a drug-sensitive phenotype via EVs has also been reported. Additionally, interactions between non-tumor cells and the tumor cells with an impact on MDR are presented. Finally, we highlight uninvestigated aspects and possible approaches to exploiting this knowledge toward the identification of druggable processes and molecules and, ultimately, the development of novel therapeutic strategies.

HSF1 Pathway Inhibitor Clinical Candidate (CCT361814/NXP800) Developed from a Phenotypic Screen as a Potential Treatment for Refractory Ovarian Cancer and Other Malignancies

CCT251236 1, a potent chemical probe, was previously developed from a cell-based phenotypic high-throughput screen (HTS) to discover inhibitors of transcription mediated by HSF1, a transcription factor that supports malignancy. Owing to its activity against models of refractory human ovarian cancer, 1 was progressed into lead optimization. The reduction of P-glycoprotein efflux became a focus of early compound optimization; central ring halogen substitution was demonstrated by matched molecular pair analysis to be an effective strategy to mitigate this liability. Further multiparameter optimization led to the design of the clinical candidate, CCT361814/NXP800 22, a potent and orally bioavailable fluorobisamide, which caused tumor regression in a human ovarian adenocarcinoma xenograft model with on-pathway biomarker modulation and a clean in vitro safety profile. Following its favorable dose prediction to human, 22 has now progressed to phase 1 clinical trial as a potential future treatment for refractory ovarian cancer and other malignancies.

The effect and mechanistic study of encequidar on reversing the resistance of SW620/AD300 cells to doxorubicin

Encequidar, a gut-specific P-glycoprotein (P-gp) inhibitor, makes oral paclitaxel possible, and has been used in clinical treatment of metastatic breast cancer, however, its pharmacological effect and mechanism of reversal of drug resistance in drug-resistant colon cancer cells SW620/AD300 are still unknown. Herein, we first synthesized encequidar and demonstrated that it could inhibit the transport activity of P-gp, reduced doxorubicin (DOX) efflux, enhanced DOX cytotoxicity and promoted tumor-apoptosis in SW620/AD300 cells. Metabolomic analysis of cell samples was performedusing liquid chromatography Q-Exactive mass spectrometer, the results of metabolite enrichment analysis and pathway analysis showed that the combination of encequidar and DOX could: i) significantly affect the citric acid cycle (TCA cycle) and reduce the energy supply required for P-gp to exert its transport activity; ii) affect the metabolism of glutathione, which is the main component of the anti-oxidative stress system, and reduce the ability of cells to resist oxidative stress; iii) increase the intracellular reactive oxygen species (ROS) production and enhance ROS-induced cell damage and lipid peroxidation, which in turn restore the sensitivity of drug-resistant cells to DOX. In conclusion, these results provide sufficient data support for the therapeutical application of the P-gp inhibitor encequidar to reverse MDR, and are of great significance to further understand the therapeutic advantages of encequidar in anti-tumor therapy and guide clinical rational drug use.

Advances in understanding the role of P-gp in doxorubicin resistance: Molecular pathways, therapeutic strategies, and prospects

P-glycoprotein (P-gp) is a drug efflux transporter that triggers doxorubicin (DOX) resistance. In this review, we highlight the molecular avenues regulating P-gp, such as Nrf2, HIF-1α, miRNAs, and long noncoding (lnc)RNAs, to reveal their participation in DOX resistance. These antitumor compounds and genetic tools synergistically reduce P-gp expression. Furthermore, ATP depletion impairs P-gp activity to enhance the antitumor activity of DOX. Nanoarchitectures, including liposomes, micelles, polymeric nanoparticles (NPs), and solid lipid nanocarriers, have been developed for the co-delivery of DOX with anticancer compounds and genes enhancing DOX cytotoxicity. Surface modification of nanocarriers, for instance with hyaluronic acid (HA), can promote selectivity toward cancer cells. We discuss these aspects with a focus on P-gp expression and activity.

Lapatinib and poziotinib overcome ABCB1-mediated paclitaxel resistance in ovarian cancer

Conventional frontline treatment for ovarian cancer consists of successive chemotherapy cycles of paclitaxel and platinum. Despite the initial favorable responses for most patients, chemotherapy resistance frequently leads to recurrent or refractory disease. New treatment strategies that circumvent or prevent mechanisms of resistance are needed to improve ovarian cancer therapy. We established in vitro paclitaxel-resistant ovarian cancer cell line and organoid models. Gene expression differences in resistant and sensitive lines were analyzed by RNA sequencing. We manipulated candidate genes associated with paclitaxel resistance using siRNA or small molecule inhibitors, and then screened the cells for paclitaxel sensitivity using cell viability assays. We used the Bliss independence model to evaluate the anti-proliferative synergy for drug combinations. ABCB1 expression was upregulated in paclitaxel-resistant TOV-21G (q < 1x10-300), OVCAR3 (q = 7.4x10-156) and novel ovarian tumor organoid (p = 2.4x10-4) models. Previous reports have shown some tyrosine kinase inhibitors can inhibit ABCB1 function. We tested a panel of tyrosine kinase inhibitors for the ability to sensitize resistant ABCB1-overexpressing ovarian cancer cell lines to paclitaxel. We observed synergy when we combined poziotinib or lapatinib with paclitaxel in resistant TOV-21G and OVCAR3 cells. Silencing ABCB1 expression in paclitaxel-resistant TOV-21G and OVCAR3 cells reduced paclitaxel IC50 by 20.7 and 6.2-fold, respectively. Furthermore, we demonstrated direct inhibition of paclitaxel-induced ABCB1 transporter activity by both lapatinib and poziotinib. In conclusion, lapatinib and poziotinib combined with paclitaxel synergizes to inhibit the proliferation of ABCB1-overexpressing ovarian cancer cells in vitro. The addition of FDA-approved lapatinib to second-line paclitaxel therapy is a promising strategy for patients with recurrent ovarian cancer.

Paclitaxel-Based Chemotherapy Targeting Cancer Stem Cells from Mono- to Combination Therapy

Paclitaxel (PTX) is a chemotherapeutical agent commonly used to treat several kinds of cancer. PTX is known as a microtubule-targeting agent with a primary molecular mechanism that disrupts the dynamics of microtubules and induces mitotic arrest and cell death. Simultaneously, other mechanisms have been evaluated in many studies. Since the anticancer activity of PTX was discovered, it has been used to treat many cancer patients and has become one of the most extensively used anticancer drugs. Regrettably, the resistance of cancer to PTX is considered an extensive obstacle in clinical applications and is one of the major causes of death correlated with treatment failure. Therefore, the combination of PTX with other drugs could lead to efficient therapeutic strategies. Here, we summarize the mechanisms of PTX, and the current studies focusing on PTX and review promising combinations.

Basic physiology of the blood-brain barrier in health and disease: a brief overview

The blood-brain barrier (BBB), a dynamic interface between blood and brain constituted mainly by endothelial cells of brain microvessels, robustly restricts the entry of potentially harmful blood-sourced substances and cells into the brain, however, many therapeutically active agents concurrently cannot gain access into the brain at effective doses in the presence of an intact barrier. On the other hand, breakdown of BBB integrity may involve in the pathogenesis of various neurodegenerative diseases. Besides, certain diseases/disorders such as Alzheimer's disease, hypertension, and epilepsy are associated with varying degrees of BBB disruption. In this review, we aim to highlight the current knowledge on the cellular and molecular composition of the BBB with special emphasis on the major transport pathways across the barrier type endothelial cells. We further provide a discussion on the innovative brain drug delivery strategies in which the obstacle formed by BBB interferes with effective pharmacological treatment of neurodegenerative diseases/disorders.

Effect of Sepatronium Bromide (YM-155) on DNA Double-Strand Breaks Repair in Cancer Cells

Survivin, as an antiapoptotic protein often overexpressed in cancer cells, is a logical target for potential cancer treatment. By overexpressing survivin, cancer cells can avoid apoptotic cell death and often become resistant to treatments, representing a significant obstacle in modern oncology. A survivin suppressor, an imidazolium-based compound known as YM-155, is nowadays studied as an attractive anticancer agent. Although survivin suppression by YM-155 is evident, researchers started to report that YM-155 is also an inducer of DNA damage introducing yet another anticancer mechanism of this drug. Moreover, the concentrations of YM-155 for DNA damage induction seems to be far lower than those needed for survivin inhibition. Understanding the molecular mechanism of action of YM-155 is of vital importance for modern personalized medicine involving the selection of responsive patients and possible treatment combinations. This review focuses mainly on the documented effects of YM-155 on DNA damage signaling pathways. It summarizes up to date literature, and it outlines the molecular mechanism of YM-155 action in the context of the DNA damage field.

Comparing the immunogenicity of glycosidase-directed resiquimod prodrugs mediated by cancer cell metabolism

We have recently developed an enzyme-directed immunostimulant (EDI) prodrug motif, which is metabolized to active immunostimulant by cancer cells and, following drug efflux, activates nearby immune cells, resulting in immunogenicity. In this study, we synthesized several EDI prodrugs featuring an imidazoquinoline immunostimulant resiquimod (a Toll-like receptor 7/8 agonist) covalently modified with glycosidase enzyme-directing groups selected from substrates of β-glucuronidase, α-mannosidase, or β-galactosidase. We compared the glycosidase-dependent immunogenicity elicited by each EDI in RAW-Blue macrophages following conversion to active immunostimulant by complementary glycosidase. At a cellular level, we examined EDI metabolism across three cancer cell lines (B16 melanoma, TC2 prostate, and 4T1 breast cancer). Comparing the relative immunogenicity elicited by each EDI/cancer cell combination, we found that B16 cells produced the highest EDI prodrug immunogenicity, achieving >95% of that elicited by unmodified resiquimod, followed by TC2 and 4T1 cells (40% and 30%, respectively). Immunogenicity elicited was comparable for a given cell type and independent of the glycosidase substrate in the EDIs or differences in functional glycosidase activity between cell lines. Measuring drug efflux of the immunostimulant payload and efflux protein expression revealed that EDI/cancer cell-mediated immunogenicity was governed by efflux potential of the cancer cells. We determined that, following EDI conversion, immunostimulant efflux occurred through both P-glycoprotein-dependent and P-glycoprotein-independent transport mechanisms. Overall, this study highlights the broad ability of EDIs to couple immunogenicity to the metabolism of many cancers that exhibit drug efflux and suggests that designing future generations of EDIs with immunostimulant payloads that are optimized for drug efflux could be particularly beneficial.

Antiproliferative Flavanoid Dimers Isolated from Brazilian Red Propolis

Herein reported are results of the chemical and biological investigation of red propolis collected at the Brazilian Northeast coastline. New propolones A-D (1-4), with a 3-{3-[(2-phenylbenzofuran-3-yl)methyl]phenyl}chromane skeleton; propolonones A-C (5-7), with a 3-[3-(3-benzylbenzofuran-2-yl)phenyl]chromane skeleton; and propolol A (8), with a 6-(3-benzylbenzofuran-2-yl)-3-phenylchromane skeleton, were isolated as constituents of Brazilian red propolis by cytotoxicity-guided assays and structurally identified by analysis of their spectroscopic data. Propolone B (2) and propolonone A (5) display significant cytotoxic activities against an ovarian cancer cell line expressing a multiple drug resistance phenotype when compared with doxorubicin.

A review on myricetin as a potential therapeutic candidate for cancer prevention

Myricetin, one of the most extensively studied polyphenols, is present abundantly in various fruits and vegetables and exhibits diverse pharmacological properties. The multifaceted biological action of myricetin against tumor heterogeneity makes it an impressive anticancer agent whose efficacy has been confirmed by an overwhelming number of studies. Myricetin shows its therapeutic potential by targeting and modulating the expression of various molecular target which are involved in inflammation, cell proliferation, apoptosis, angiogenesis, invasion, and metastasis. Myricetin deters tumor progression by inducing apoptosis via both intrinsic and extrinsic pathway, activating/inactivating several signaling pathways, and reactivating various tumor suppressor genes. This comprehensive review represents the effect of myricetin on various hallmarks of cancer with insight into the molecular mechanism employed by myricetin to mitigate cell proliferation, angiogenesis, metastasis, and induce apoptosis. In addition, enhanced bioavailability of myricetin through conjugation and its increased efficacy as an anticancer agent when used in combination are also highlighted.

Improving Treatment Efficacy of In Situ Forming Implants via Concurrent Delivery of Chemotherapeutic and Chemosensitizer

P-glycoprotein (Pgp), a member of the ATP-binding cassette family, is one of the major causes of multidrug resistance in tumors. Current clinical treatments to overcome MDR involve the co-delivery of a Pgp inhibitor and a chemotherapeutic. A concern for this treatment that has led to varied clinical trial success is the associated systemic toxicities involving endogenous Pgp. Local drug delivery systems, such as in situ forming implants (ISFIs), alleviate this problem by delivering a high concentration of the drug directly to the target site without the associated systemic toxicities. ISFIs are polymeric drug solutions that undergo a phase transition upon injection into an aqueous environment to form a solid drug eluting depot allowing for a high initial intratumoral drug concentration. In this study, we have developed an ISFI capable of overcoming the Pgp resistance by co-delivering a chemotherapeutic, Doxorubicin (Dox), with a Pgp inhibitor, either Pluronic P85 or Valspodar (Val). Studies investigated in vitro cytotoxicity of Dox when combined with either Pgp inhibitor, effect of the inhibitors on release of Dox from implants in PBS, in vivo Dox distribution and retention in a subcutaneous flank colorectal murine tumor, and therapeutic response characterized by tumor growth curves and histopathology. Dox + Val showed a 4-fold reduction in the 50% lethal dose (LD₅₀) after 48 hours. Concurrent delivery of Dox and Val showed the greatest difference at 16 days post injection for both Dox penetration and retention. This treatment group had a 5-fold maximum Dox penetration compared to Dox alone ISFIs (0.53 ± 0.22 cm vs 0.11 ± 0.11 cm, respectively, from the center of the ISFI). Additionally, there was a 3-fold increase in normalized total intratumoral Dox intensity with the Dox + Val ISFIs compared to Dox alone ISFIs (0.54 ± 0.11 vs 0.18 ± 0.09, respectively). Dox + Val ISFIs showed a 2-fold reduction in tumor growth and a 27.69% increase in necrosis 20 days post-injection compared to Dox alone ISFIs. These findings demonstrate that co-delivery of Dox and Val via ISFI can avoid systemic toxicity issues seen with clinical Pgp inhibitors.

Dual P-Glycoprotein and CA XII Inhibitors: A New Strategy to Reverse the P-gp Mediated Multidrug Resistance (MDR) in Cancer Cells

A new series of N,N-bis(alkanol)amine aryl diesters was synthesized and studied as dual P-glycoprotein (P-gp) and carbonic anhydrase XII inhibitors (CA XII). These hybrids should be able to synergistically overcome P-gp mediated multidrug resistance (MDR) in cancer cells. It was reported that the efflux activity of P-gp could be modulated by CA XII, as the pH reduction caused by CA XII inhibition produces a significant decrease in P-gp ATPase activity. The new compounds reported here feature both P-gp and CA XII binding moieties. These hybrids contain a N,N-bis(alkanol)amine diester scaffold found in P-glycoprotein ligands and a coumarin or benzene sulfonamide moiety to target CA XII. Many compounds displayed a dual activity against P-gp and CA XII being active in the Rhd 123 uptake test on K562/DOX cells and in the hCA XII inhibition test. On LoVo/DOX cells, that overexpress both P-gp and CA XII, some coumarin derivatives showed a high MDR reversal effect in Rhd 123 uptake and doxorubicin cytotoxicity enhancement tests. In particular, compounds 7 and 8 showed higher activity than verapamil and were more potent on LoVo/DOX than on K562/DOX cells overexpressing only P-gp. They can be considered as valuable candidates for selective P-gp/CA XII inhibition in MDR cancer cells.

The primary mechanism of cytotoxicity of the chemotherapeutic agent CX-5461 is topoisomerase II poisoning

Small molecules can affect many cellular processes. The disambiguation of these effects to identify the causative mechanisms of cell death is extremely challenging. This challenge impacts both clinical development and the interpretation of chemical genetic experiments. CX-5461 was developed as a selective RNA polymerase I inhibitor, but recent evidence suggests that it may cause DNA damage and induce G-quadraplex formation. Here we use three complimentary data mining modalities alongside biochemical and cell biological assays to show that CX-5461 exerts its primary cytotoxic activity through topoisomerase II poisoning. We then show that acquired resistance to CX-5461 in previously sensitive lymphoma cells confers collateral resistance to the topoisomerase II poison doxorubicin. Doxorubicin is already a frontline chemotherapy in a variety of hematopoietic malignancies, and CX-5461 is being tested in relapse/refractory hematopoietic tumors. Our data suggest that the mechanism of cell death induced by CX-5461 is critical for rational clinical development in these patients. Moreover, CX-5461 usage as a specific chemical genetic probe of RNA polymerase I function is challenging to interpret. Our multimodal data-driven approach is a useful way to detangle the intended and unintended mechanisms of drug action across diverse essential cellular processes.

Analysis of determinants for in vitro resistance to the small molecule deubiquitinase inhibitor b-AP15

BACKGROUND

b-AP15/VLX1570 are small molecule inhibitors of the ubiquitin specific peptidase 14 (USP14) and ubiquitin carboxyl-terminal hydrolase 5 (UCHL5) deubiquitinases (DUBs) of the 19S proteasome. b-AP15/VLX1570 have been shown to be cytotoxic to cells resistant to bortezomib, raising the possibility that this class of drugs can be used as a second-line therapy for treatment-resistant multiple myeloma. Limited information is available with regard to potential resistance mechanisms to b-AP15/VLX1570.

RESULTS

We found that b-AP15-induced cell death is cell-cycle dependent and that non-cycling tumor cells may evade b-AP15-induced cell death. Such non-cycling cells may re-enter the proliferative state to form colonies of drug-sensitive cells. Long-term selection of cells with b-AP15 resulted in limited drug resistance (~2-fold) that could be reversed by buthionine sulphoximine, implying altered glutathione (GSH) metabolism as a resistance mechanism. In contrast, drug uptake and overexpression of drug efflux transporters were found not to be associated with b-AP15 resistance.

CONCLUSIONS

The proteasome DUB inhibitors b-AP15/VLX1570 are cell cycle-active. The slow and incomplete development of resistance towards these compounds is an attractive feature in view of future clinical use.

A High-Throughput Screen of a Library of Therapeutics Identifies Cytotoxic Substrates of P-glycoprotein

The ATP-binding cassette transporter P-glycoprotein (P-gp) is known to limit both brain penetration and oral bioavailability of many chemotherapy drugs. Although US Food and Drug Administration guidelines require that potential interactions of investigational drugs with P-gp be explored, often this information does not enter the literature. In response, we developed a high-throughput screen to identify substrates of P-gp from a series of chemical libraries, testing a total of 10,804 compounds, most of which have known mechanisms of action. We used the CellTiter-Glo viability assay to test library compounds against parental KB-3-1 human cervical adenocarcinoma cells and the colchicine-selected subline KB-8-5-11 that overexpresses P-gp. KB-8-5-11 cells were also tested in the presence of a P-gp inhibitor (tariquidar) to assess reversibility of transporter-mediated resistance. Of the tested compounds, a total of 90 P-gp substrates were identified, including 55 newly identified compounds. Substrates were confirmed using an orthogonal killing assay against human embryonic kidney-293 cells overexpressing P-gp. We confirmed that AT7159 (cyclin-dependent kinase inhibitor), AT9283, (Janus kinase 2/3 inhibitor), ispinesib (kinesin spindle protein inhibitor), gedatolisib (PKI-587, phosphoinositide 3-kinase/mammalian target of rampamycin inhibitor), GSK-690693 (AKT inhibitor), and KW-2478 (heat-shock protein 90 inhibitor) were substrates. In addition, we assessed direct ATPase stimulation. ABCG2 was also found to confer high levels of resistance to AT9283, GSK-690693, and gedatolisib, whereas ispinesib, AT7519, and KW-2478 were weaker substrates. Combinations of P-gp substrates and inhibitors were assessed to demonstrate on-target synergistic cell killing. These data identified compounds whose oral bioavailability or brain penetration may be affected by P-gp. SIGNIFICANCE STATEMENT: The ATP-binding cassette transporter P-glycoprotein (P-gp) is known to be expressed at barrier sites, where it acts to limit oral bioavailability and brain penetration of substrates. In order to identify novel compounds that are transported by P-gp, we developed a high-throughput screen using the KB-3-1 cancer cell line and its colchicine-selected subline KB-8-5-11. We screened the Mechanism Interrogation Plate (MIPE) library, the National Center for Advancing Translational Science (NCATS) pharmaceutical collection (NPC), the NCATS Pharmacologically Active Chemical Toolbox (NPACT), and a kinase inhibitor library comprising 977 compounds, for a total of 10,804 compounds. Of the 10,804 compounds screened, a total of 90 substrates were identified of which 55 were novel. P-gp expression may adversely affect the oral bioavailability or brain penetration of these compounds.

Identification of anticancer OATP2B1 substrates by an in vitro triple-fluorescence-based cytotoxicity screen

Membrane transporters play an important role in the absorption, distribution, metabolism and excretion of drugs. The cellular accumulation of many drugs is the result of the net function of efflux and influx transporters. Efflux transporters such as P-glycoprotein/ABCB1 have been shown to confer multidrug resistance in cancer. Although expression of uptake transporters has been confirmed in cancer cells, their role in chemotherapy response has not been systematically investigated. In the present study we have adapted a fluorescence-based cytotoxic assay to characterize the influence of key drug-transporters on the toxicity of approved anticancer drugs. Co-cultures of fluorescently labeled parental and transporter-expressing cells (expressing ABCB1, ABCG2 or OATP2B1) were screened against 101 FDA-approved anticancer drugs, using a novel, automated, triple fluorescence-based cytotoxicity assay. By measuring the survival of parental and transporter-expressing cells in co-cultures, we identify those FDA-approved anticancer drugs, whose toxicity is influenced by ABCB1, ABCG2 or OATP2B1. In addition to confirming known substrates of ABCB1 and ABCG2, the fluorescence-based cytotoxicity assays identified anticancer agents whose toxicity was increased in OATP2B1 expressing cells. Interaction of these compounds with OATP2B1 was verified in dedicated transport assays using cell-impermeant fluorescent substrates. Understanding drug-transporter interactions is needed to increase the efficacy of chemotherapeutic agents. Our results highlight the potential of the fluorescence-based HT screening system for identifying transporter substrates, opening the way for the design of therapeutic approaches based on the inhibition or even the exploitation of transporters in cancer cells.

Next generation diversity-oriented synthesis: a paradigm shift from chemical diversity to biological diversity

Diversity-oriented synthesis adds biological performance as a new diversity element.

Magnetic Resonance-Guided Focused Ultrasound : Current Status and Future Perspectives in Thermal Ablation and Blood-Brain Barrier Opening

Magnetic resonance-guided focused ultrasound (MRgFUS) is an emerging new technology with considerable potential to treat various neurological diseases. With refinement of ultrasound transducer technology and integration with magnetic resonance imaging guidance, transcranial sonication of precise cerebral targets has become a therapeutic option. Intensity is a key determinant of ultrasound effects. High-intensity focused ultrasound can produce targeted lesions via thermal ablation of tissue. MRgFUS-mediated stereotactic ablation is non-invasive, incision-free, and confers immediate therapeutic effects. Since the US Food and Drug Administration approval of MRgFUS in 2016 for unilateral thalamotomy in medication-refractory essential tremor, studies on novel indications such as Parkinson's disease, psychiatric disease, and brain tumors are underway. MRgFUS is also used in the context of blood-brain barrier (BBB) opening at low intensities, in combination with intravenously-administered microbubbles. Preclinical studies show that MRgFUS-mediated BBB opening safely enhances the delivery of targeted chemotherapeutic agents to the brain and improves tumor control as well as survival. In addition, BBB opening has been shown to activate the innate immune system in animal models of Alzheimer's disease. Amyloid plaque clearance and promotion of neurogenesis in these studies suggest that MRgFUS-mediated BBB opening may be a new paradigm for neurodegenerative disease treatment in the future. Here, we review the current status of preclinical and clinical trials of MRgFUS-mediated thermal ablation and BBB opening, described their mechanisms of action, and discuss future prospects.

Overcoming chemoresistance in prostate cancer with Chinese medicine Tripterygium wilfordii via multiple mechanisms

A leading cause of cancer chemotherapy failure is chemoresistance, which often involves multiple mechanisms. Chinese medicines (CM) usually contain multiple components which could potentially target many mechanisms simultaneously and may offer an advantage over single compounds that target one mechanism at a time. The purpose of this study was to investigate the chemosensitizing effect (CE) of a specific CM, Tripterygium wilfordii (TW), on prostate cancer cells resistant to docetaxel (Dtx) and identify the potential mechanisms. The CE of TW (in combination with Dtx) was evaluated in two Dtx resistant prostate cancer cell lines (PC3-TxR and DU145-TxR) and the efficacy of the combination for resistant PC3-TxR tumor was investigated using a xenograft mouse model. For mechanistic study, the inhibitory effect of TW on P-glycoprotein activity was assessed. In addition, novel gene targets of TW were identified using DNA microarray and quantitative PCR. Results showed that TW induced a CE of 8 and >38 folds in PC3-TxR and DU145-TxR cells, respectively with Dtx IC50 reversed back to that of the sensitive parent cells. An optimum dose of TW+Dtx significantly retarded tumor growth in mice compared to TW or Dtx alone. TW inhibited P-glycoprotein activity and induced a significant gene expression changes in genes related to angiogenesis, cell cycle regulation and differentiation. Our in vitro and in vivo studies demonstrate that TW in combination with Dtx was able to overcome the chemoresistance and suppress resistant prostate tumor growth via multi-mechanisms.

Multifactorial Modes of Action of Arsenic Trioxide in Cancer Cells as Analyzed by Classical and Network Pharmacology

Arsenic trioxide is a traditional remedy in Chinese Medicine since ages. Nowadays, it is clinically used to treat acute promyelocytic leukemia (APL) by targeting PML/RARA. However, the drug's activity is broader and the mechanisms of action in other tumor types remain unclear. In this study, we investigated molecular modes of action by classical and network pharmacological approaches. CEM/ADR5000 resistance leukemic cells were similar sensitive to As2O3 as their wild-type counterpart CCRF-CEM (resistance ratio: 1.88). Drug-resistant U87.MG ΔEGFR glioblastoma cells harboring mutated epidermal growth factor receptor were even more sensitive (collateral sensitive) than wild-type U87.MG cells (resistance ratio: 0.33). HCT-116 colon carcinoma p53-/- knockout cells were 7.16-fold resistant toward As2O3 compared to wild-type cells. Forty genes determining cellular responsiveness to As2O3 were identified by microarray and COMPARE analyses in 58 cell lines of the NCI panel. Hierarchical cluster analysis-based heat mapping revealed significant differences between As2O3 sensitive cell lines and resistant cell lines with p-value: 1.86 × 10-5. The genes were subjected to Galaxy Cistrome gene promoter transcription factor analysis to predict the binding of transcription factors. We have exemplarily chosen NF-kB and AP-1, and indeed As2O3 dose-dependently inhibited the promoter activity of these two transcription factors in reporter cell lines. Furthermore, the genes identified here and those published in the literature were assembled and subjected to Ingenuity Pathway Analysis for comprehensive network pharmacological approaches that included all known factors of resistance of tumor cells to As2O3. In addition to pathways related to the anticancer effects of As2O3, several neurological pathways were identified. As arsenic is well-known to exert neurotoxicity, these pathways might account for neurological side effects. In conclusion, the activity of As2O3 is not restricted to acute promyelocytic leukemia. In addition to PML/RARA, numerous other genes belonging to diverse functional classes may also contribute to its cytotoxicity. Network pharmacology is suited to unravel the multifactorial modes of action of As2O3.

Inhibit or Evade Multidrug Resistance P-Glycoprotein in Cancer Treatment

Multidrug resistance (MDR) is a major cause of failure in cancer chemotherapy. P-glycoprotein (P-gp), a promiscuous drug efflux pump, has been extensively studied for its association with MDR due to overexpression in cancer cells. Several P-gp inhibitors or modulators have been investigated in clinical trials in hope of circumventing MDR, with only limited success. Alternative strategies are actively pursued, such as the modification of existing drugs, development of new drugs, or combination of novel drug delivery agents to evade P-gp-dependent efflux. Despite the importance and numerous studies, these efforts have mostly been undertaken without a priori knowledge of how drugs interact with P-gp at the molecular level. This review highlights and discusses progress toward and challenges impeding drug development for inhibiting or evading P-gp in the context of our improved understanding of the structural basis and mechanism of P-gp-mediated MDR.

Functional characterization and anti-cancer action of the clinical phase II cardiac Na+/K+ ATPase inhibitor istaroxime: in vitro and in vivo properties and cross talk with the membrane androgen receptor

Sodium potassium pump (Na⁺/K⁺ ATPase) is a validated pharmacological target for the treatment of various cardiac conditions. Recent published data with Na⁺/K⁺ ATPase inhibitors suggest a potent anti-cancer action of these agents in multiple indications. In the present study, we focus on istaroxime, a Na⁺/K⁺ ATPase inhibitor that has shown favorable safety and efficacy properties in cardiac phase II clinical trials. Our experiments in 22 cancer cell lines and in prostate tumors in vivo proved the strong anti-cancer action of this compound. Istaroxime induced apoptosis, affected the key proliferative and apoptotic mediators c-Myc and caspase-3 and modified actin cystoskeleton dynamics and RhoA activity in prostate cancer cells. Interestingly, istaroxime was capable of binding to mAR, a membrane receptor mediating rapid, non-genomic actions of steroids in prostate and other cells. These results support a multi-level action of Na⁺/K⁺ ATPase inhibitors in cancer cells and collectively validate istaroxime as a strong re-purposing candidate for further cancer drug development.

Sensitivity of docetaxel-resistant MCF-7 breast cancer cells to microtubule-destabilizing agents including vinca alkaloids and colchicine-site binding agents

Introduction

One of the main reasons for disease recurrence in the curative breast cancer treatment setting is the development of drug resistance. Microtubule targeted agents (MTAs) are among the most commonly used drugs for the treatment of breaset cancer and therefore overcoming taxane resistance is of primary clinical importance. Our group has previously demonstrated that the microtubule dynamics of docetaxel-resistant MCF-7_TXT cells are insensitivity to docetaxel due to the distinct expression profiles of β-tubulin isotypes in addition to the high expression of p-glycoprotein (ABCB1). In the present investigation we examined whether taxane-resistant breast cancer cells are more sensitive to microtubule destabilizing agents including vinca alkaloids and colchicine-site binding agents (CSBAs) than the non-resistant cells.

Methods

Two isogenic MCF-7 breast cancer cell lines were selected for resistance to docetaxel (MCF-7_TXT) and the wild type parental cell line (MCF-7_CC) to examine if taxane-resistant breast cancer cells are sensitive to microtubule-destabilizing agents including vinca alkaloids and CSBAs. Cytotoxicity assays, immunoblotting, indirect immunofluorescence and live imaging were used to study drug resistance, apoptosis, mitotic arrest, microtubule formation, and microtubule dynamics.

Results

MCF-7_TXT cells were demonstrated to be cross resistant to vinca alkaloids, but were more sensitive to treatment with colchicine compared to parental non-resistant MCF-7_CC cells. Cytotoxicity assays indicated that the IC₅₀ of MCF-7_TXT cell to vinorelbine and vinblastine was more than 6 and 3 times higher, respectively, than that of MCF-7_CC cells. By contrast, the IC₅₀ of MCF-7_TXT cell for colchincine was 4 times lower than that of MCF-7_CC cells. Indirect immunofluorescence showed that all MTAs induced the disorganization of microtubules and the chromatin morphology and interestingly each with a unique pattern. In terms of microtubule and chromain morphology, MCF-7_TXT cells were more resistant to vinorelbine and vinblastine, but more sensitive to colchicine compared to MCF-7_CC cells. PARP cleavage assay further demonstrated that all of the MTAs induced apoptosis of the MCF-7 cells. However, again, MCF-7_TXT cells were more resistant to vinorelbine and vinblastine, and more sensitive to colchicine compared to MCF-7_CC cells. Live imaging demonstrated that the microtubule dynamics of MCF-7_TXT cells were less sensitive to vinca alkaloids, and more sensitive to colchicine. MCF-7_TXT cells were also noted to be more sensitive to other CSBAs including 2MeOE₂, ABT-751 and phosphorylated combretastatin A-4 (CA-4P).

Conclusion

Docetaxel-resistant MCF-7_TXT cells have demonstrated cross-resistance to vinca alkaloids, but appear to be more sensitive to CSBAs (colchicine, 2MeOE₂, ABT-751 and CA-4P) compared to non-resistant MCF-7_CC cells. Taken together these results suggest that CSBAs should be evaluated further in the treatment of taxane resistant breast cancer.

Sphingolipid abnormalities in cancer multidrug resistance: Chicken or egg?

The cancer multidrug resistance (MDR) phenotype encompasses a myriad of molecular, genetic and cellular alterations resulting from progressive oncogenic transformation and selection. Drug efflux transporters, in particular the MDR P-glycoprotein ABCB1, play an important role in MDR but cannot confer the complete phenotype alone indicating parallel alterations are prerequisite. Sphingolipids are essential constituents of lipid raft domains and directly participate in functionalization of transmembrane proteins, including providing an optimal lipid microenvironment for multidrug transporters, and are also perturbed in cancer. Here we postulate that increased sphingomyelin content, developing early in some cancers, recruits and functionalizes plasma membrane ABCB1 conferring a state of partial MDR, which is completed by glycosphingolipid disturbance and the appearance of intracellular vesicular ABCB1. In this review, the independent and interdependent roles of sphingolipid alterations and ABCB1 upregulation during the transformation process and resultant conferment of partial and complete MDR phenotypes are discussed.

Myricetin induces apoptosis and enhances chemosensitivity in ovarian cancer cells

Ovarian cancer is the most lethal type of gynecological cancer and is the fifth leading cause of cancer-associated mortality in females globally. The majority of patients with ovarian cancer suffer from recurrent, progressive disease, due to the acquisition of a resistance phenotype towards various conventional chemotherapy drugs. Although paclitaxel has been demonstrated to be effective against ovarian tumors, there have been reports of the development of a resistant phenotype against Taxol^® treatment. The multidrug resistance (MDR)-1/P-glycoprotein has previously been demonstrated to be associated with the acquisition of paclitaxel resistance in certain ovarian tumors. Therefore, the screening of novel drug candidates able to target MDR-1 in ovarian cancer cells and increase the sensitivity to Taxol^® is required in order to improve the treatment of this disease. In the present study, the underlying mechanisms by which the dietary flavonoid myricetin enhances the cytotoxic potential of paclitaxel in ovarian cancer cells, was investigated. It was observed that myricetin induced significant cytotoxicity in A2780 and OVCAR3 ovarian cancer cells, with the IC50 value obtained at 25 µM. Myricetin treatment also resulted in the induction of apoptosis in the two cell lines, accompanied by the modulation of certain pro- and anti-apoptotic markers. It was also determined that the pre-incubation of ovarian cancer cells with a lower dose of myricetin was able to increase the cytotoxicity of paclitaxel, due to the significant downregulation of MDR-1 in these cells.

Regorafenib overcomes chemotherapeutic multidrug resistance mediated by ABCB1 transporter in colorectal cancer: In vitro and in vivo study

Chemotherapeutic multidrug resistance (MDR) is a significant challenge to overcome in clinic practice. Several mechanisms contribute to MDR, one of which is the augmented drug efflux induced by the upregulation of ABCB1 in cancer cells. Regorafenib, a multikinase inhibitor targeting the RAS/RAF/MEK/ERK pathway, was approved by the FDA to treat metastatic colorectal cancer and gastrointestinal stromal tumors. We investigated whether and how regorafenib overcame MDR mediated by ABCB1. The results showed that regorafenib reversed the ABCB1-mediated MDR and increased the accumulation of [³H]-paclitaxel in ABCB1-overexpressing cells by suppressing efflux activity of ABCB1, but not altering expression level and localization of ABCB1. Regorafenib inhibited ATPase activity of ABCB1. In mice bearing resistant colorectal tumors, regorafenib raised the intratumoral concentration of paclitaxel and suppressed the growth of resistant colorectal tumors. But regorafenib did not induce cardiotoxicity/myelosuppression of paclitaxel in mice. Strategy to reposition one FDA-approved anticancer drug regorafenib to overcome the resistance of another FDA-approved, widely used chemotherapeutic paclitaxel, may be a promising direction for the field of adjuvant chemotherapy. This study provides clinical rationale for combination of conventional chemotherapy and targeted anticancer agents.

A subset of platinum-containing chemotherapeutic agents kills cells by inducing ribosome biogenesis stress

Cisplatin and its platinum analogs, carboplatin and oxaliplatin, are some of the most widely used cancer chemotherapeutics. Although cisplatin and carboplatin are used primarily in germ cell, breast and lung malignancies, oxaliplatin is instead used almost exclusively to treat colorectal and other gastrointestinal cancers. Here we utilize a unique, multi-platform genetic approach to study the mechanism of action of these clinically established platinum anti-cancer agents, as well as more recently developed cisplatin analogs. We show that oxaliplatin, unlike cisplatin and carboplatin, does not kill cells through the DNA-damage response. Rather, oxaliplatin kills cells by inducing ribosome biogenesis stress. This difference in drug mechanism explains the distinct clinical implementation of oxaliplatin relative to cisplatin, and it might enable mechanistically informed selection of distinct platinum drugs for distinct malignancies. These data highlight the functional diversity of core components of front-line cancer therapy and the potential benefits of applying a mechanism-based rationale to the use of our current arsenal of anti-cancer drugs.

Using Pharmacogenomic Databases for Discovering Patient-Target Genes and Small Molecule Candidates to Cancer Therapy

With multiple omics strategies being applied to several cancer genomics projects, researchers have the opportunity to develop a rational planning of targeted cancer therapy. The investigation of such numerous and diverse pharmacogenomic datasets is a complex task. It requires biological knowledge and skills on a set of tools to accurately predict signaling network and clinical outcomes. Herein, we describe Web-based in silico approaches user friendly for exploring integrative studies on cancer biology and pharmacogenomics. We briefly explain how to submit a query to cancer genome databases to predict which genes are significantly altered across several types of cancers using CBioPortal. Moreover, we describe how to identify clinically available drugs and potential small molecules for gene targeting using CellMiner. We also show how to generate a gene signature and compare gene expression profiles to investigate the complex biology behind drug response using Connectivity Map. Furthermore, we discuss on-going challenges, limitations and new directions to integrate molecular, biological and epidemiological information from oncogenomics platforms to create hypothesis-driven projects. Finally, we discuss the use of Patient-Derived Xenografts models (PDXs) for drug profiling in vivo assay. These platforms and approaches are a rational way to predict patient-targeted therapy response and to develop clinically relevant small molecules drugs.

Bis-cyclopropane analog of disorazole C1 is a microtubule-destabilizing agent active in ABCB1-overexpressing human colon cancer cells

The novel, chemically stabilized disorazole analog, (−)-CP₂-disorazole C₁ (1) displayed potent anti-proliferative activity against a broad-spectrum of human colorectal cancer cells. HCT15 and H630R1 cell lines expressing high basal levels of the ABCB1 protein, known to cause multi-drug resistance, were also sensitive to growth inhibition by 1 but were resistant to both vincristine and docetaxel, two commonly used microtubule inhibitors. Compound 1 exhibited strong inhibition of tubulin polymerization at a level comparable to vincristine. In addition, treatment with 1 resulted in decreased protein levels of β-tubulin but not α-tubulin. An analysis of cellular proteins known to interact with microtubules showed that 1 caused decreased expression of c-Myc, APC, Rb, and additional key cellular signaling pathways in CRC cells. Treatment with compound 1 also resulted in G2/M cell cycle arrest and induction of apoptosis, but not senescence. Furthermore, endothelial spheroid sprouting assays demonstrated that 1 suppressed angiogenesis and can, therefore, potentially prevent cancer cells from spreading and metastasizing. Taken together, these findings suggest that the microtubule disruptor 1 may be a potential drug candidate for the treatment of mCRC.

Mechanisms of breast cancer resistance to anthracyclines or taxanes: an overview of the proposed roles of noncoding RNA

PURPOSE OF REVIEW

Anthracyclines and taxanes are the two most active classes of cytotoxic agents that are commonly used for the treatment of breast cancer. However, resistance to these agents has become a major clinical obstacle. The aim of the present review is to define the roles of noncoding RNA (ncRNA) in breast cancer progression and the development of chemotherapy resistance. The ultimate goal is to exploit ncRNAs as new therapeutic tools to overcome resistance.

RECENT FINDINGS

Two important types of ncRNA include microRNA (miRNA) and long noncoding RNA (lncRNA). Both miRNA and lncRNA have recently impacted the field of breast cancer research as important pieces in the mechanistic puzzle of the genes and pathways involved in breast cancer development and progression.

SUMMARY

Herein, we review the roles of miRNA and lncRNA in breast cancer progression and the development of chemotherapy resistance. Future research should include identification of ncRNAs that could be potential therapeutic targets in chemotherapy-resistant tumors, as well as ncRNA biomarkers that facilitate more tumor-specific treatment options for chemotherapy-resistant breast cancer patients.

Combining ABCG2 Inhibitors with IMMU-132, an Anti-Trop-2 Antibody Conjugate of SN-38, Overcomes Resistance to SN-38 in Breast and Gastric Cancers

Sacituzumab govitecan (IMMU-132), an SN-38-conjugated antibody-drug conjugate, is showing promising therapeutic results in a phase I/II trial of patients with advanced Trop-2-expressing, metastatic, solid cancers. As members of the ATP-binding cassette (ABC) transporters confer chemotherapy resistance by active drug efflux, which is a frequent cause of treatment failure, we explored the use of known inhibitors of ABC transporters for improving the therapeutic efficacy of IMMU-132 by overcoming SN-38 resistance. Two human tumor cell lines made resistant to SN-38, MDA-MB-231-S120 (human breast cancer) and NCI-N87-S120 (human gastric cancer), were established by continuous exposure of the parental cells to stepwise increased concentrations of SN-38 and analyzed by flow cytometry for functional activities of ABCG2 and ABCB1, immunoblotting and qRT-PCR for the expression of ABCG2 at both protein and mRNA levels, and MTS assays for the potency of SN-38 alone or in combination with a modulator of ABC transporters. MDA-MB-231-S120 and NCI-N87-S120 displayed reduced sensitivity to SN-38 in vitro, with IC50 values approximately 50-fold higher than parental MDA-MB-231 and NCI-N87 cells. The increase in drug resistance of both S120 cell populations is associated with the expression of functional ABCG2, but not ABCB1. Importantly, treatment of both S120 sublines with known ABCG2 inhibitors (fumitremorgin C, Ko143, and YHO-13351) restored toxicity of SN-38, and the combination of YHO-13351 with IMMU-132 increased the median survival of mice bearing NCI-N87-S120 xenografts. These results provide a rationale for combination therapy of IMMU-132 and inhibitors of ABC transporters, such as YHO-13351. Mol Cancer Ther; 15(8); 1910-9. ©2016 AACR.

NCI-60 Cell Line Screening: A Radical Departure in its Time

The year 2015 marked the 30th anniversary of National Cancer Institute's (NCI's) initial effort to establish a human cell line panel as the basis for discovering new cancer drugs. At its inception, the NCI-60 panel was a controversial departure, born of frustration with previous efforts that employed murine tumors and grounded in the hope that the biology of human tumors was diverse and somehow quite different than the murine leukemia used for the previous 30 years. And while the NCI-60 has not revolutionized cancer drug discovery in terms of the new drugs that resulted, it represents a turning point in the philosophy and practice of cancer drug research.

Improved in vitro human tumor models for cancer gene therapy

Developing effective anticancer treatments is a particular challenge, as agents must contend with not only the target cellular biology, but also with the complex tumor microenvironment. Here we discuss various in vitro strategies that have sought to address this issue, with a particular focus on new methodologies that utilize clinical samples in basic research and their application in gene therapy and virotherapy.

The modulation of ABC transporter-mediated multidrug resistance in cancer: a review of the past decade

ATP-binding cassette (ABC) transporters represent one of the largest and oldest families of membrane proteins in all extant phyla from prokaryotes to humans, which couple the energy derived from ATP hydrolysis essentially to translocate, among various substrates, toxic compounds across the membrane. The fundamental functions of these multiple transporter proteins include: (1) conserved mechanisms related to nutrition and pathogenesis in bacteria, (2) spore formation in fungi, and (3) signal transduction, protein secretion and antigen presentation in eukaryotes. Moreover, one of the major causes of multidrug resistance (MDR) and chemotherapeutic failure in cancer therapy is believed to be the ABC transporter-mediated active efflux of a multitude of structurally and mechanistically distinct cytotoxic compounds across membranes. It has been postulated that ABC transporter inhibitors known as chemosensitizers may be used in combination with standard chemotherapeutic agents to enhance their therapeutic efficacy. The current paper reviews the advance in the past decade in this important domain of cancer chemoresistance and summarizes the development of new compounds and the re-evaluation of compounds originally designed for other targets as transport inhibitors of ATP-dependent drug efflux pumps.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

Characterization of proapoptotic compounds from the bark of Garcinia oblongifolia

Twenty compounds from Garcinia oblongifolia were screened for proapoptotic activity using FRET-based HeLa-C3 sensor cells. Among them, oblongifolins F and G (1 and 2), 1,3,5-trihydroxy-13,13-dimethyl-2H-pyran[7,6-b]xanthone (3), nigrolineaxanthone T (4), and garcicowin B (5) showed significant proapoptotic activity at a concentration of 10 μM. Bioassessments were then performed to evaluate the potential of these compounds for therapeutic application. All five compounds showed significant cytotoxicity and caspase-3-activating ability in cervical cancer HeLa cells, with compounds 1 and 2 having the highest potencies. All five compounds specifically induced caspase-dependent apoptosis, which could be prevented by the pan-caspase inhibitor zVAD-fmk. In particular, 3 induced apoptosis through mitotic arrest. Compounds 1-5 displayed similar IC50 values (3.9-16.5 μM) against the three cancer cell lines HeLa, MDA-MB-435, and HepG2. In addition, compounds 1, 2, and 4 exhibited similar and potent IC50 values (2.4-5.1 μM) against several breast and colon cancer cell lines, including those overexpressing either HER2 or P-glycoprotein. HER2 and P-glycoprotein are known factors that confer resistance to anticancer drugs in cancer cells. This is the first study on the cytotoxicity, caspase-3-activing ability, and specificity of proapoptotic compounds isolated from G. oblongifolia in HeLa cells. The potential application of these compounds against HER2- or P-glycoprotein-overexpressing cancer cells was investigated.

Linsitinib (OSI-906) antagonizes ATP-binding cassette subfamily G member 2 and subfamily C member 10-mediated drug resistance

In this study we investigated the effect of linsitinib on the reversal of multidrug resistance (MDR) mediated by the overexpression of the ATP-binding cassette (ABC) subfamily members ABCB1, ABCG2, ABCC1 and ABCC10. Our results indicate for the first time that linsitinib significantly potentiate the effect of anti-neoplastic drugs mitoxantrone (MX) and SN-38 in ABCG2-overexpressing cells; paclitaxel, docetaxel and vinblastine in ABCC10-overexpressing cells. Linsitinib moderately enhanced the cytotoxicity of vincristine in cell lines overexpressing ABCB1, whereas it did not alter the cytotoxicity of substrates of ABCC1. Furthermore, linsitinib significantly increased the intracellular accumulation and decreased the efflux of [(3)H]-MX in ABCG2-overexpressing cells and [(3)H]-paclitaxel in ABCC10-overexpressing cells. However, linsitinib, at a concentration that reversed MDR, did not significantly alter the expression levels of either the ABCG2 or ABCC10 transporter proteins. Furthermore, linsitinib did not significantly alter the intracellular localization of ABCG2 or ABCC10. Moreover, linsitinib stimulated the ATPase activity of ABCG2 in a concentration-dependent manner. Overall, our study suggests that linsitinib attenuates ABCG2- and ABCC10-mediated MDR by directly inhibiting their function as opposed to altering ABCG2 or ABCC10 protein expression.

Complex Interplay between the P-Glycoprotein Multidrug Efflux Pump and the Membrane: Its Role in Modulating Protein Function

Multidrug resistance in cancer is linked to expression of the P-glycoprotein multidrug transporter (Pgp, ABCB1), which exports many structurally diverse compounds from cells. Substrates first partition into the bilayer and then interact with a large flexible binding pocket within the transporter's transmembrane regions. Pgp has been described as a hydrophobic vacuum cleaner or an outwardly directed drug/lipid flippase. Recent X-ray crystal structures have shed some light on the nature of the drug-binding pocket and suggested routes by which substrates can enter it from the membrane. Detergents have profound effects on Pgp function, and several appear to be substrates. Biochemical and biophysical studies in vitro, some using purified reconstituted protein, have explored the effects of the membrane environment. They have demonstrated that Pgp is involved in a complex relationship with its lipid environment, which modulates the behavior of its substrates, as well as various functions of the protein, including ATP hydrolysis, drug binding, and drug transport. Membrane lipid composition and fluidity, phospholipid headgroup and acyl chain length all influence Pgp function. Recent studies focusing on thermodynamics and kinetics have revealed some important principles governing Pgp-lipid and substrate-lipid interactions, and how these affect drug-binding and transport. In some cells, Pgp is associated with cholesterol-rich microdomains, which may modulate its functions. The relationship between Pgp and cholesterol remains an open question; however, it clearly affects several aspects of its function in addition to substrate-membrane partitioning. The action of Pgp modulators appears to depend on their membrane permeability, and membrane fluidizers and surfactants reverse drug resistance, likely via an indirect mechanism. A detailed understanding of how the membrane affects Pgp substrates and Pgp's catalytic cycle may lead to new strategies to combat clinical drug resistance.

Multiple mechanisms underlying acquired resistance to taxanes in selected docetaxel-resistant MCF-7 breast cancer cells

Background

Chemoresistance is a major factor involved in a poor response and reduced overall survival in patients with advanced breast cancer. Although extensive studies have been carried out to understand the mechanisms of chemoresistance, many questions remain unanswered.

Methods

In this research, we used two isogenic MCF-7 breast cancer cell lines selected for resistance to doxorubicin (MCF-7_DOX) or docetaxel (MCF-7_TXT) and the wild type parental cell line (MCF-7_CC) to study mechanisms underlying acquired resistance to taxanes in MCF-7_TXT cells. Cytotoxicity assay, immunoblotting, indirect immunofluorescence and live imaging were used to study the drug resistance, the expression levels of drug transporters and various tubulin isoforms, apoptosis, microtubule formation, and microtubule dynamics.

Results

MCF-7_TXT cells were cross resistant to paclitaxel, but not to doxorubicin. MCF-7_DOX cells were not cross-resistant to taxanes. We also showed that multiple mechanisms are involved in the resistance to taxanes in MCF-7_TXT cells. Firstly, MCF-7_TXT cells express higher level of ABCB1. Secondly, the microtubule dynamics of MCF-7_TXT cells are weak and insensitive to the docetaxel treatment, which may partially explain why docetaxel is less effective in inducing M-phase arrest and apoptosis in MCF-7_TXT cells in comparison with MCF-7_CC cells. Moreover, MCF-7_TXT cells express relatively higher levels of β2- and β4-tubulin and relatively lower levels of β3-tubulin than both MCF-7_CC and MCF-7_DOX cells. The subcellular localization of various β-tubulin isoforms in MCF-7_TXT cells is also different from that in MCF-7_CC and MCF-7_DOX cells.

Conclusion

Multiple mechanisms are involved in the resistance to taxanes in MCF-7_TXT cells. The high expression level of ABCB1, the specific composition and localization of β-tubulin isoforms, the weak microtubule dynamics and its insensitivity to docetaxel may all contribute to the acquired resistance of MCF-7_TXT cells to taxanes.

A novel microtubule inhibitor 4SC-207 with anti-proliferative activity in taxane-resistant cells

Microtubule inhibitors are invaluable tools in cancer chemotherapy: taxanes and vinca alkaloids have been successfully used in the clinic over the past thirty years against a broad range of tumors. However, two factors have limited the effectiveness of microtubule inhibitors: toxicity and resistance. In particular, the latter is highly unpredictable, variable from patient to patient and is believed to be the cause of treatment failure in most cases of metastatic cancers. For these reasons, there is an increasing demand for new microtubule inhibitors that can overcome resistance mechanisms and that, at the same time, have reduced side effects. Here we present a novel microtubule inhibitor, 4SC-207, which shows strong anti-proliferative activity in a large panel of tumor cell lines with an average GI50 of 11 nM. In particular, 4SC-207 is active in multi-drug resistant cell lines, such as HCT-15 and ACHN, suggesting that it is a poor substrate for drug efflux pumps. 4SC-207 inhibits microtubule growth in vitro and in vivo and promotes, in a dose dependent manner, a mitotic delay/arrest, followed by apoptosis or aberrant divisions due to chromosome alignment defects and formation of multi-polar spindles. Furthermore, preliminary data from preclinical studies suggest low propensity towards bone marrow toxicities at concentrations that inhibit tumor growth in paclitaxel-resistant xenograft models. In summary, our results suggest that 4SC-207 may be a potential anti-cancer agent.

The network of P-glycoprotein and microRNAs interactions

Overexpression of P-glycoprotein (P-gp) contributes to the multidrug resistance (MDR) phenotype found in many cancer cells. P-gp has been identified as a promising molecular target, although attempts to find successful therapies to counteract its function as a drug efflux pump have largely failed to date. Apart from its role in drug efflux, P-gp may have other cellular functions such as being involved in apoptosis, and is found in various locations in the cell. Its expression is highly regulated, namely by microRNAs (miRNAs or miRs). In addition, P-gp may regulate the expression of miRs in the cell. Furthermore, both P-gp and miRs may be found in microvesicles or exosomes and may be transported to neighboring, drug-sensitive cells. Here, we review this current issue together with recent evidence of this network of interactions between P-gp and miRs.

Accurate models for P-gp drug recognition induced from a cancer cell line cytotoxicity screen

P-glycoprotein (P-gp, MDR1) is a promiscuous drug efflux pump of substantial pharmacological importance. Taking advantage of large-scale cytotoxicity screening data involving 60 cancer cell lines, we correlated the differential biological activities of ∼13,000 compounds against cellular P-gp levels. We created a large set of 934 high-confidence P-gp substrates or nonsubstrates by enforcing agreement with an orthogonal criterion involving P-gp overexpressing ADR-RES cells. A support vector machine (SVM) was 86.7% accurate in discriminating P-gp substrates on independent test data, exceeding previous models. Two molecular features had an overarching influence: nearly all P-gp substrates were large (>35 atoms including H) and dense (specific volume of <7.3 Å(3)/atom) molecules. Seven other descriptors and 24 molecular fragments ("effluxophores") were found enriched in the (non)substrates and incorporated into interpretable rule-based models. Biological experiments on an independent P-gp overexpressing cell line, the vincristine-resistant VK2, allowed us to reclassify six compounds previously annotated as substrates, validating our method's predictive ability. Models are freely available at http://pgp.biozyne.com .